It has been known that, in a die-sinking electrical discharge machining apparatus that machines a workpiece by generating pulse discharge in a machining gap between a machining electrode and the workpiece facing each other, a discharge state during electrical discharge machining can be determined by detecting a high-frequency component in a discharge voltage and judging the size of the high-frequency component. Patent Document 1, for example, discloses such a technology for judging the discharge state from a high-frequency component.
FIG. 8 is a circuit diagram of relevant part of an electrical discharge machining apparatus substantively identical in configuration to that described in Patent Document 1. A pulsed discharge voltage is supplied to a machining gap between an electrode 2 of the electrical discharge machining apparatus and a workpiece 3. A high-pass filter 4 extracts a high-frequency component from the discharge voltage. A rectifier 5 rectifies the high-frequency component extracted by the high-pass filter 4 and outputs the rectified high-frequency component as an output signal Vrec. A discharge voltage detecting device 75 detects the discharge voltage at the machining gap between the electrode 2 and the workpiece 3. A discharge current detecting device 76 detects a discharge current at the machining gap between the electrode 2 and the workpiece 3.
An output signal u from the discharge voltage detecting device 75 and an output signal i from the discharge current detecting device 76 are input to a logic circuit 77. A time constant measuring device 70 and a logic circuit 72 constitute a delay circuit. The time constant measuring device 70 measures a time constant tH of the high-pass filter 4. An output signal 79 from the logic circuit 77 is input to the time constant measuring device 70 and the logic circuit 72. An output signal 71 from the time constant measuring device 70 is input to the logic circuit 72. An integrator circuit 9 includes a capacitor C1 and a resistor R1. The capacitor C1 is connected between an inverting (−) input side and an output side of an operational amplifier. The resistor R1 is serially connected between an output side of the rectifier 5 and the inverting (−) input side of the operational amplifier. A non-inverting (+) input side of the operational amplifier is grounded.
A reset circuit 10 includes a transistor. A collector-emitter of the transistor is connected between both terminals of the capacitor C1. An output signal 73 from the logic circuit 72 is input to the reset circuit 10. An integrated output value Vint that is an output signal from the operational amplifier of the integrator circuit 9 is input to an inverting (−) input side of a comparator 78. A reference voltage Vref is input to a non-inverting (+) input side of the comparator 78.
FIG. 9 is a timing chart of input and output signal waveforms in the electrical discharge machining apparatus shown in FIG. 8. A waveform A is a discharge voltage waveform at the machining gap between the electrode 2 and the workpiece 3. A waveform B is an output signal waveform from the high-pass filter 4. A waveform G is an output signal waveform from the logic circuit 77. A waveform H is an output signal waveform from the time constant measuring device 70. A waveform I is an output signal waveform from the logic circuit 72. A waveform F is an integrated output signal waveform from the integrator circuit 9.
Next, the operation is described with reference to FIGS. 8 and 9. In FIG. 9, a waveform 80 is the discharge voltage waveform at the machining gap between the electrode 2 and the workpiece 3. A time interval Ton indicates a discharge-pulse width. A time interval Toff indicates pulse off time. After a voltage is applied to the machining gap between the electrode 2 and the workpiece 3, an electrical discharge is generated. When the electrical discharge is generated, levels of output signals from the discharge voltage detecting device 75 and the discharge current detecting device 76 are both high (H). The output signals are input to the logic circuit 77. When the levels of both signals input to the logic circuit 77 are H or, in other words, when the electrical discharge is generated in the machining gap between the electrode 2 and the workpiece 3, the logic circuit 77 outputs a low (L) level signal. A time at which the logic circuit 77 outputs the L level signal is a discharge detection time t1. Time t2 is a time (t2=t1+tH) after the time constant tH of the high-pass filter 4, with the discharge detection time t1 as a starting point.
A waveform 82 indicates the high-frequency component of the discharge voltage. A waveform 83 indicates a disturbance waveform due to a transient characteristic of the high-pass filter 4. The time constant measuring device 70 outputs an H level signal during the time tH, with a time at which the output signal 79 from the logic circuit 77 falls as a starting point (H in FIG. 9). The output signal 79 from the logic circuit 77 and the output signal 71 from the time constant measuring device 70 are input to the logic circuit 72. The logic circuit 72 outputs the output signal 73, as indicated in I in FIG. 9. A time at which the output signal 73 falls is indicated by t2 in I in FIG. 9. The reset circuit 10 resets the integrator circuit 9 while a level of the output signal 73 from the logic circuit 72 is H. In other words, the integrator circuit 9 integrates the output signal Vrec from the rectifier 5 only while the level of the output signal 73 from the logic circuit 72 is L. The comparator 78 compares the reference voltage Vref with the integrated output Vint, indicated in F in FIG. 9. When the integrated output Vint is larger than the reference voltage Vref at an end of the discharge-pulse width Ton, the comparator 78 judges the discharge pulse to be a normal discharge pulse. When an opposite is true, the comparator 78 judges the discharge pulse to be an abnormal discharge pulse, such as an arc discharge pulse.
It has been known that the discharge state of the electrical discharge machining apparatus during electrical discharge machining can be judged by a detection of a discharge voltage level. Patent Document 2, for example, discloses such a technology for judging the discharge state from the discharge voltage level.
FIG. 10 is a circuit diagram of relevant part of an electrical discharge machining apparatus substantively identical in configuration to that described in Patent Document 2. In FIG. 10, like reference characters refer to portions corresponding to those shown in FIG. 8, and explanations thereof are omitted. A machining pulse generating circuit includes a machining power supply 1, an upstream resistor 100, and a switch 90. The switch 90 is controlled by a Schmitt trigger circuit 91, a first monostable flip-flop 92, a second monostable flip-flop 93, and an AND gate 94. The Schmitt trigger circuit 91 is used to detect the generation of the electrical discharge after the voltage is applied to the machining gap. The first monostable flip-flop 92 is used to fix the discharge-pulse width Ton. The second monostable flip-flop 93 is used to fix the pulse off time Toff of an interval between two discharge voltage pulses. One inputting unit of the AND gate 94 is connected to the flip-flop 93. Another inputting unit of the AND gate 94 is connected to a control circuit. The control circuit includes two comparators 95 and 96. The control circuit compares an upper threshold V2 and a lower threshold V1 of a voltage, serving as reference voltage values, with the discharge voltage at the machining gap. When a measured voltage is included in a middle of two reference voltages, an AND gate 97 sends one output signal during an elapse of time F, fixed by a monostable flip-flop 98.
FIG. 11 is a schematic diagram for explaining a relation between various discharge voltage waveforms 80 of the electrical discharge machining apparatus shown in FIG. 10 and current waveforms 84 based on a duration F of a voltage-read window. A waveform A is the discharge voltage waveform. A waveform B is an output signal waveform 85 of the duration F of the read window during which the voltage level of the discharge voltage waveform 80 is detected. A waveform C is the discharge current waveform 84. A form A1 and a form A2 of the discharge voltage waveform are higher than the upper threshold V2 (for example, 20 volts) of the discharge voltage. The pulse width of the discharge voltage and the current are held during the discharge-pulse width Ton. A form B1 and a form B2 of the discharge voltage waveform are lower, than the upper threshold V2 of the discharge voltage and higher than the lower threshold V1 (for example, 5 volts) of the voltage. The pulse width of the discharge voltage and the current are cut off after reading is completed. A form C of the discharge voltage waveform is lower than the lower threshold V1. The pulse width of the discharge voltage and the current are held during the discharge voltage width Ton.
A technology has been known in which the discharge state is improved by controlling the pulse off time and machining conditions are controlled to increase machining efficiency when the discharge pulse is judged to be abnormal. Such a technology is disclosed in, for example, Patent Document 1.
FIG. 12 is a circuit diagram of relevant part of another electrical discharge machining apparatus substantively identical in configuration to that described in Patent Document 1. In FIG. 12, like reference characters refer to portions corresponding to those shown in FIG. 8, and explanations thereof are omitted. The electrical discharge machining apparatus includes a short-circuit detecting device 28, a first comparator 29, a comparison-reference-value generating device 30, a second comparator 31, and a second comparison value generating device 32.
FIG. 13 is a timing chart of input and output signal waveforms in relevant part of a power-supply control device shown in FIG. 12. A waveform A is the discharge voltage waveform at the machining gap between the electrode 2 and the workpiece 3. A waveform B is the output signal waveform from the high-pass filter 4. A waveform C is an output signal waveform from the rectifier 5. A waveform P is an output signal waveform from a discharge detecting device 23. A waveform Q is an output signal waveform from a timer 24. A waveform F is an output signal waveform from the integrator circuit 9. A waveform S is an output signal waveform from the first comparator 29. The first comparator 29 compares an output from the discharge detecting device 23 with a first reference value. A waveform T is an output signal waveform from the second comparator 31. The second comparator 31 compares the output from the discharge detecting device 23 with a second reference value. A waveform U is an output signal waveform from the short-circuit detecting device 28.
Next, the operation is described with reference to FIGS. 12 and 13. The machining power supply 1 applies a pulsed voltage to the machining gap, and electrical discharge machining is performed. The high-pass filter 4 extracts only the high-frequency component from the discharge voltage waveform A output when the electrical discharge machining is performed. The extracted high-frequency component becomes the output signal waveform B. The rectifier 5 rectifies the acquired high-frequency component. The rectified high-frequency component becomes the output signal waveform C. The output signal waveform C is input to the integrator circuit 9. When the electrical discharge is generated, the output signal waveform P from the discharge detecting device 23 rises. The integrator circuit 9 is reset, and the timer 24 starts. The output signal waveform P becomes the output signal waveform Q. The integrator circuit 9 integrates the output signal waveform C. The output signal waveform C becomes the output signal waveform F. When the electrical discharge ends, a discharge detection output from the discharge detecting device 23 falls. With the fall of the discharge detection output, the first comparator 29 and the second comparator 31 output comparison results of a comparison of reference values from the first comparison-reference-value generating device 30 and the second comparison-reference-value generating device 32 (set lower than the first reference value) with the output from the integrator circuit 9. The first comparator 29 and the second comparator 31 output the comparison results as the output signal waveform S and the output signal waveform T. As a result, the discharge pulse is classified into three types that are the normal discharge pulse, a quasi-arc discharge pulse, and the arc discharge pulse.
The short-circuit detecting device 28 checks a voltage value at the machining gap when the discharge detection output falls. The short-circuit detecting device 28 compares the voltage value with a short-circuit reference voltage (preferably 15 volts or less) and outputs a short-circuit detection signal (the waveform U in FIG. 8). A short-circuited state described herein refers not only to a direct connection between the electrode 2 and the workpiece 3, but also a short-circuit via tar into which machining scraps and machining fluid have transformed, carbide film formed on an electrode surface, or the like. Therefore, the voltage value at the machining gap cannot be completely zero volts. A voltage of about several volts is generated. The short-circuit reference voltage differs depending on an electrode material, as does an arc discharge voltage. Therefore, there is no standard value for all cases. The short-circuit reference voltage is, for example, set to 15 volts or less and preferably 10 volts or less, when the electrode 2 is copper and the workpiece 3 is steel.
A machining-condition control device 27 acquires three types of identification outputs from the first comparator 29 and the second comparator 31. The three types are the normal discharge, the quasi-arc discharge, and the arc discharge. The machining-condition control device 27 also acquires two types of identification outputs from the short-circuit detecting device 28, indicating whether the short-circuited state is present. The machining-condition control device 27 acquires the identification outputs for each pulse. Therefore, the machining-condition control device 27 acquires classified identification outputs. When the short-circuited state is present, the machining-condition control device 27 does not change machining conditions. In the case of quasi-arc discharge, the machining-condition control device 27 switches the pulse off time to longer one. In the case of arc discharge, the machining-condition control device 27 switches the pulse off time to further longer one. In the case of non-short-circuited normal discharge, the machining-condition control device 27 shortens the pulse off time. As described above, in addition to recognition of the discharge state through whether the high-frequency component is present, the discharge pulse and the presence of the short-circuited state are judged, and the machining conditions is controlled. As a result, an optimum machining state is maintained.    Patent Document 1: Japanese Patent Application Laid-open No. H5-293714    Patent Document 2: Japanese Patent Application Laid-open No. S61-159326